Method of calibration of a stereoscopic camera system for use with a radio therapy treatment apparatus

ABSTRACT

The disclosed calibration method includes a calibration phantom positioned on an adjustable table on the surface of a mechanical couch, with the phantom&#39;s centre at an estimated location for the iso-centre of a radio therapy treatment apparatus. The calibration phantom is then irradiated using the apparatus, and the relative location of the center of the calibration phantom and the iso-centre of the apparatus is determined by analyzing images of the irradiation of the calibration phantom. The calibration phantom is then repositioned by the mechanical couch applying an offset corresponding to the determined relative location of the centre of the calibration phantom and the iso-centre of the apparatus to the calibration phantom. Images of the relocated calibration phantom are obtained, to which the offset has been applied, and the obtained images are processed to set the co-ordinate system of a stereoscopic camera system relative to the iso-centre of the apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/899,112 filed on Dec. 16, 2015, which is a National Phase filingunder 35 C.F.R. § 371 of and claims priority to PCT Patent ApplicationNo.: PCT/GB2014/052125, filed on Jul. 11, 2014, which claims thepriority benefit under 35 U.S.C. § 119 of British Application No.:1312808.7, filed on Jul. 17, 2013, the contents of which each of whichare hereby incorporated in their entireties by reference.

BACKGROUND

The present invention concerns a method of calibration of a stereoscopiccamera system. In particular, embodiments of the present inventionconcern a method of calibration of a stereoscopic camera system for usewith a radio therapy treatment apparatus.

Radiotherapy consists of projecting, onto a predetermined region of apatient's body, a radiation beam so as to destroy or eliminate tumoursexisting therein. Such treatment is usually carried out periodically andrepeatedly. At each medical intervention, the radiation source must bepositioned with respect to the patient in order to irradiate theselected region with the highest possible accuracy to avoid radiatingadjacent tissue on which radiation beams would be harmful.

A fundamental problem with radiotherapy is the need to position thepatient in the same position, when obtaining diagnostic images and eachof the subsequent times when radiation is applied to the patient's body.It is to that end that Vision RT have developed an image processingsystem for monitoring and positioning patients described in U.S. Pat.Nos.: U.S. Pat. No. 7,348,974, U.S. Pat. No. 7,889,906, U.S. Pat. No.8,135,201, and pending U.S. patent application Ser. No. 12/379,108,published as U.S. 2009/0187112.

In use, in the Vision RT system, images of a patient on a mechanicalcouch are obtained by a set of stereoscopic cameras which are thenprocessed to generate a 3D wire mesh model of the surface of a patientbeing monitored. This 3D wire mesh model is compared with a referencesurface created during treatment planning. The relative positioning ofthe model and the reference surface is compared and used to generateinstructions for the mechanical couch to position the couch, vertically,laterally and rotationally so as to match the surfaces and hence locatethe patient reliably in the same location relative to the iso-centre ofa treatment apparatus. Subsequently during treatment the position of apatient is continually monitored and if for any reason the patient movesor repositions themselves, this can be detected and appropriate actioncan be taken if necessary.

There are several sources of uncertainty in radio therapy treatmentsystems such as errors in patient positioning, target localization, anddose delivery. It is practically impossible to achieve perfect alignmentmainly due to the presence of several geometric errors in the system.One of the critical geometric errors in radio therapy treatments isuncertainty in localizing the radiation field centre, which directlyaffects the dosimetric accuracy and results in incorrect tumourtargeting that may lead to the delivery of inadequate dose to the lesionand/or serious damage to the healthy adjacent tissues. Therefore, it isnecessary to develop methods to reduce the probability of such errors byextensive and efficient quality assurance programs to ensure high-levelgeometric accuracy of the treatment.

Originally, the primary method for iso-centre verification inradiotherapy was to measure the distance between the tip of a mechanicalpointer mounted on the gantry head of a treatment apparatus and a fixedpoint mounted on the treatment table. Such a method was manual,laborious and time-consuming. The accuracy of the method depended uponthe human observer and was also limited by size of the tip of pointerused.

An improved technique was introduced by Lutz, Winston and Maleki atHarvard Medical School in 1988 which is described in Lutz W, Winston KR, Maleki N. A system for stereotactic radiosurgery with a linearaccelerator. Int J Radiat Oncol Biol Phys. 1988; 14(2):373-81. In theWinston-Lutz system a calibration phantom comprising a small metallicball made of steel, titanium or tungsten is fixed on the treatment tableby a locking mechanism. The phantom position is adjustable in threedirections by means of a micrometer tool. The collimator used forradiotherapy is attached to the gantry head and the ball is placed asclosely as possible to the iso-centre by aligning the marks on thephantom with the treatment room lasers. The collimated beam is used toexpose a radiographic test film mounted perpendicular to the beamdirection on a stand behind the ball. Differences between the centre ofthe sphere shadow and the field centre identifies the differencesbetween the true iso-centre and the iso-centre as indicated by thetreatment room lasers. The offset is read on each film using transparenttemplate guidance scales or scanning the film and software analysis.

A mathematical method for analysing Winston Lutz images was developedand is described in Low D A, Li Z, Drzymala R E. Minimization of targetpositioning error in accelerator-based radiosurgery. Med Phys. 1995;22(4):443-48 which used the film-measured iso-centre positional errorsfor eight gantry angle and couch settings to find the suitable offsetfor the phantom stand to minimize the distance between the treatmentapparatus iso-centre and the target. A similar aim was followed by Grimmet al., who developed an algorithm to reconstruct the Winston-Lutzphantom ball locus in three dimensions from two-dimensional film imagestaken at certain couch and gantry angles and combined them with theimages of lasers taken by digital cameras. This approach is described inGrimm J, Grimm S L, Das I J, et al. A quality assurance method withsub-millimetre accuracy for stereotactic linear accelerators. J ApplClin Med Phys. 2011; 12(1):182-98.

A further example of automated processing of phantom images is describedin E Schriebmann, E Elder and T Fox, Automatied Quality Assurance forImage-Guided Radiation Therapy, J Appl Clin Med Phys. 2009:10(1):71-79which discusses the automation of Quality Assurance methods to ensurethat a megavoltage (MV) treatment beam coincides with an integratedkilovoltage (kV) or volumetric cone beam CT. In the paper, a calibrationcube is described as being located at the estimated location oftreatment room iso-centre using laser markings. Images of theirradiation of the cube are then obtained and processed to determine theextent the cube as positioned is offset from the iso-centre asidentified by the MV, kV and volumetric cone beams.

Calibration of stereoscopic camera systems for use in treatment roomshas developed alongside methods for identifying the iso-centre of atreatment apparatus. Calibration techniques used to calibrate the VisionRT patient monitoring system are described in U.S. Pat. No. 7,348,974and U.S. Pat. No. 7,889,906.

As is described in U.S. Pat. No. 7,348,974 and U.S. Pat. No. 7,889,906 acalibration sheet comprising a 40*40 cm sheet of flat rigid materialsuch as aluminium or steel on which a pattern revealing a 20*20 matrixof circles at known positions on the surface of the sheet is provided.Additionally, towards the centre of the calibration sheet are foursmaller markers adjacent to four circles the centres of which togetheridentify the four corners of a square of known size.

Images of the calibration sheet are obtained and processed to identifywithin the image the positions of the four markers in the images andtheir associated circles. From the relative positions of circlesidentified by the markers in the images, a projective transformation isdetermined which accounts for the estimated centres of the identifiedcircles defining the corners of a parallelogram in the image whicharises due to the relative orientation of the calibration sheet and thecamera obtaining the image. The calculated transform is then applied toeach of the identified circles in turn to transform the oval shapes ofthe circles. More accurate estimates of the positions of the centres ofthe four circles are then determined by identifying the centres of thetransformed circles and utilising an inverse transform to determine thecorresponding position of the estimated circle centre in the originalimage.

When the co-ordinates for all the centres of each of the representationsof the circles on the calibration sheet have been calculated for animage, the relative orientation of the different cameras can then becalculated from the relative positions of these points in the images andthe known relative locations of these circles on the surface of thecalibration sheet as is described in detail in “A Versatile CameraCalibration Technique for High-Accuracy 3D Machine Vision MetrologyUsing Off the Shelf TV Cameras and Lenses”, Roger Tsai, IEEE Journal ofRobotics and Automation, Vol. Ra-3, No. 4, August 1987. Further from therelative positions of the points in the individual images internalcamera parameters such as the focal length and radial distortion withinthe camera images can also be determined.

Having determined the locations of the stereoscopic cameras, and anyradial distortions present in the camera images, the positioning of thecameras relative to the iso-centre of the treatment apparatus is thendetermined. This is achieved by imaging a calibration cube of known sizewhich is positioned on a treatment apparatus at a position with itscentre at the iso-centre of the treatment apparatus as indicated by theco-incidence of marks on the exterior of the cube with the projection ofthe laser cross hairs which intersect at the iso-centre.

The images of the calibration cube are processed utilising thepreviously obtained measurements of the relative locations of thecameras and any data about the existence of any distortion present inthe images to generate a 3D computer model of the surface of the cube.Since the cube has known dimensions and is at a known location and in aknown orientation relative to the iso-centre of the treatment apparatusas indicated by the laser cross-hairs, a comparison between thegenerated 3D model and the known parameters for the size and position ofthe calibration cube enables measurements made in the co-ordinate systemof the modelling software to be converted into real world measurementsin the treatment room relative to the treatment iso-centre.

Although the conventional approach to calibrating a stereoscopic camerasystem for use with a radio therapy treatment apparatus is highlyaccurate, further improvements in accuracy are desirable.

SUMMARY

In accordance with one aspect of the present invention there is provideda method of calibration of a stereoscopic camera system for use with aradio therapy treatment apparatus.

In the method, initially a calibration phantom is positioned with thephantom's centre at an estimated location for the iso-centre of a radiotherapy treatment apparatus. The calibration phantom is then irradiatedusing the radio therapy treatment apparatus. The relative location ofthe centre of the calibration phantom and the iso-centre of the radiotherapy treatment is then determined by analysing images of theirradiation of the calibration phantom. The calibration phantom is thenrepositioned, applying an offset corresponding to the determinedrelative location of the centre of the calibration phantom and theiso-centre of the radio therapy treatment apparatus to the calibrationphantom. The stereoscopic camera system then proceeds to obtain imagesof the relocated calibration phantom to which the offset has beenapplied, processing the images to set the co-ordinate system of thestereoscopic camera system relative to the iso-centre of the radiotherapy treatment apparatus.

Thus in this way the co-ordinate system for the stereoscopic camerasystem is established based directly on the positioning of a calibrationphantom where the calibration phantom's location is directly linked to adetermination of the iso-centre of a treatment apparatus.

In some embodiments, the stereoscopic camera system may additionally beutilised to monitor the repositioning of the calibration phantom. Thismay involve processing images of a relocated calibration phantom andcomparing a generated 3D computer model of the surface of therepositioned calibration phantom with a 3D computer model of theexpected surface of the calibration phantom positioned at the iso-centreof the treatment apparatus.

In such embodiments the 3D computer model of the expected surface of thecalibration phantom positioned at the iso-centre of the treatmentapparatus may comprise a stored 3D computer model of the surface of thecalibration phantom.

Alternatively, the stereoscopic camera system may be utilised togenerate a 3D computer model of the expected surface of the calibrationphantom positioned at the iso-centre of the treatment apparatus. In sucha system initially a model of the surface of the phantom may begenerated by processing images of the calibration phantom positioned atthe estimated location for the iso-centre of the radio therapy treatmentapparatus. A model of the expected surface of the calibration phantommay then be generated by applying an offset corresponding to thedetermined relative location of the centre of the calibration phantomand the iso-centre of the radio therapy treatment apparatus to thegenerated model.

In some embodiments, a calibration phantom may be positioned at theestimated location of the iso-centre of a treatment apparatus by beingplaced on a mechanical couch. In such embodiment, relocating thecalibration phantom may be achieved by instructing the mechanical couchto offset the current location of the calibration phantom by an offsetcorresponding to the determined relative location of the centre of thecalibration phantom and the iso-centre of the radio therapy treatmentapparatus.

In some embodiments an adjustable table may be used to level acalibration phantom. The adjustable table may be placed on the surfaceof a mechanical couch. In such embodiments the calibration phantom maybe fixed on the adjustable table with the adjustable table being leveledthereby levelling the calibration phantom. In such embodiments theadjustable table may include a spirit level to facilitate levelling acalibration phantom by adjusting the adjustable table.

In embodiments of the claimed invention a calibration phantom maycomprise a calibration phantom containing one or more calibrationtargets. In embodiments where the calibration phantom contains multiplecalibration targets, preferably the calibration targets are arrangedasymmetrically within the calibration phantom so assist with theanalysis of images of an irradiated phantom.

Analysing images of the irradiation of the calibration phantom maycomprise determining the relative location of the centre of thecalibration phantom and the iso-centre of the radio therapy treatmentapparatus by comparing the locations of the one or more targets inimages of the irradiated calibration phantom irradiated with thetreatment apparatus being in a number of different positions.

In some embodiments the relocation of a calibration phantom may beundertaken iteratively. That is to say that having relocated acalibration phantom, the relocated calibration phantom may be irradiatedand imaged whilst in its new location. The relative location of thecentre of the relocated calibration phantom and the iso-centre of theradio therapy treatment apparatus can then be determined by analysingimages of the irradiation of the relocated calibration phantom.

The calibration phantom may then be further relocated by applying anoffset corresponding to the determined relative location of the centreof the relocated calibration phantom and the iso-centre of the radiotherapy treatment apparatus to the relocated calibration phantom. Theco-ordinate system of the stereoscopic camera system may then be set byprocessing obtained images of the calibration phantom as relocated.

In some embodiments multiple calibration phantoms may be used. Morespecifically, an initial estimate of the location of the iso-centre of atreatment apparatus may be obtained by irradiating a first calibrationphantom and positioning the calibration phantom on the basis of analysisof images of the irradiated calibration phantom. The first calibrationphantom could then be replaced and further adjustments of position maybe determined utilising that second calibration phantom.

In such systems initially a first phantom containing multiplecalibration targets could be used. Where such targets are arrangedasymmetrically within the body of the calibration phantom, processingimages of the irradiated phantom can identify translations and rotationsfor positioning the centre of a phantom to coincide with the iso-centreof a treatment apparatus.

Having done so, fine adjustments may then be based on the use of asecond calibration phantom using containing a single small targetpositioned in the centre of a calibration phantom thereby improvingaccuracy.

In accordance with another aspect of the present invention there isprovided a method of calibrating a stereoscopic camera system for usewith a radio therapy treatment apparatus, the method comprising:positioning a calibration phantom with the phantom's centre at anestimated location for the iso-centre of a radio therapy treatmentapparatus; utilizing a stereoscopic camera system to obtain images ofthe phantom and processing the images to generate a 3D computer model ofthe surface of the phantom; irradiating the calibration phantom usingthe radio therapy treatment apparatus and determining the relativelocation of the centre of the calibration phantom and the iso-centre ofthe radio therapy treatment apparatus by analysing images of theirradiation of the calibration phantom; and setting the co-ordinatesystem of the stereoscopic camera system on the basis of the determinedrelative location of the centre of the calibration phantom and theiso-centre of the radio therapy treatment apparatus and a determinedlocation of the centre of the modelled phantom.

In such an embodiment setting the co-ordinate system of the stereoscopiccamera system may comprise: determining the position of the centre ofthe modelled phantom relative to a current origin for the co-ordinatesystem for the 3D computer model and adjusting the co-ordinate systemfor the 3D computer model by the sum of the transformation required tomake the current origin for the co-ordinate system for the 3D computermodel coincide with the determined position of the centre of themodelled phantom and the transformation required to make location of thecentre of the calibration phantom coincident with the iso-centre of theradio therapy treatment apparatus.

The determination of the transformation required to make the currentorigin for the co-ordinate system for the 3D computer model coincidewith the determined position of the centre of the modelled phantom mayin some embodiments be determined by storing a 3D computer model of asurface of a calibration phantom with the centre of the calibrationmodel coincident with the centre of the calibration phantom; anddetermining the transformation required to make the current origin forthe co-ordinate system for the 3D computer model coincide with thedetermined position of the centre of the modelled phantom on the basisof a comparison of the modelled surface of the phantom generated on thebasis of images obtained by the stereoscopic camera with the storedmodel.

The above described systems of calibration have a number of advantagesover conventional systems for calibrating stereoscopic camera systemsfor use with radio therapy apparatus.

Conventionally, the iso-centre of an treatment apparatus is establishedusing an approach such as suggested by Winston Lutz. That location wasthen normally marked using a laser system so that intersecting planes oflaser light coincided at the identified iso-centre and laser lightsystem was then utilised to position a calibration cube for calibratinga stereoscopic camera system. However, the use of the laser light systemhas the potential to introduce additional errors into the calibrationand therefore introduce errors into the accuracy of the patientpositioning system. Further the use of a laser light system alsorequires that the accuracy with which the laser lights identify theiso-centre of a treatment apparatus must be repeatedly checked andconfirmed in case this varies over time. Instead in accordance with theclaimed invention, the calibration of a stereoscopic camera system isachieved directly through imaging and irradiating a calibration phantomwithout having to rely upon the positioning of a phantom using lasers.This both increases the accuracy of the calibration and also avoids theneed to calibrate the positioning lasers so that they correctly identifythe position of the iso-centre over time.

Further details of the present invention will become apparent withreference to the following description and drawings in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic perspective view of a stereoscopic camera systemand a treatment apparatus; and

FIG. 2 is a flow diagram of a method of calibration of a stereoscopiccamera system in accordance with the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a schematic perspective view of a treatment system. Thetreatment system includes a set of stereoscopic cameras 10 that areconnected by wiring (not shown) to a computer 12. The computer 12 isalso connected to treatment apparatus 14 such as a linear acceleratorfor applying radiotherapy. A mechanical couch 16 is provided as part ofthe treatment apparatus. The treatment apparatus 14 and the mechanicalcouch 16 are arranged such that, under the control of the computer 12,the relative positions of the mechanical couch 16 and the treatmentapparatus 14 may be varied, laterally, vertically, longitudinally androtationally as is indicated in the figure by the arrows adjacent thecouch. Additionally the treatment apparatus 14 is also able to rotateabout an axis as indicated by the arrows in the centre of the apparatusin the figure.

The computer 12 is arranged to process images obtained by thestereoscopic cameras 10 to generate 3D wire mesh computer models of thesurfaces of objects and people lying on the mechanical couch 16. Whenappropriately calibrated, a comparison of surface of a patient lying onthe couch 16 with a reference surface of the same patient generatedduring the planning stage of treatment can enable the computer 12 todetermine a transformation to match the two surfaces and generate a setof instructions to reposition the mechanical couch 16 so as to minimisedifferences between the surfaces. When repositioned, the patient can betreated using the treatment apparatus 14 in the knowledge that thepatient is in the same position as the position used to determinetreatment planning.

In order to minimise errors in the application of radiotherapy andpatient positioning it is essential that the stereoscopic camera system10 and the computer 12 are calibrated to the highest accuracy. Inparticular it is necessary that the co-ordinate system used to generate3D wire mesh models of the surface of a patient is matched both with theiso-centre of the treatment apparatus 14 and the axes of motion of themechanical couch 16. In most embodiments, the iso-centre of thetreatment apparatus 14 will be the location of the zero co-ordinates forthe modelling system and the axes of the co-ordinate system for themodelling system will be selected to correspond to the axes of motion(i.e. the lateral, vertical and horizontal axes of motion) of themechanical couch 16. This then simplifies the conversion oftransformations to match model surfaces in model space with instructionsfor relocating the mechanical couch 16 to match the position of apatient so that they can be treated using the treatment apparatus 14.

FIG. 2 is a flow diagram of a method of calibrating the treatment systemof FIG. 1.

To calibrate the system, initially (s1) the relative locations of thestereoscopic cameras in the stereoscopic camera system 10 are determinedby imaging a calibration sheet such as is described in U.S. Pat. No.7,348,974 and U.S. Pat. No. 7,889,906. That is to say that a flat rigidcalibration sheet such as a 70*70 cm sheet of material such as aluminiumor steel on which a pattern revealing a matrix of circles at knownpositions on the surface of the sheet is placed on the mechanical couchand imaged by the stereoscopic cameras 10. On the sheet in addition tothe pattern of circles are four smaller markers adjacent to four circlesthe centres of which together identify the four corners of a square ofknown size.

The images of the sheet obtained by the cameras are then processed bythe computer 12 to identify within the image the positions of the fourmarkers in the images. From the relative positions of circles identifiedby the markers in the images, for each image a first projectivetransformation is determined which accounts for the estimated centres ofthe identified circles defining the corners of a projected distortedsquare in the image which arises due to the relative orientation of thecalibration sheet and the camera obtaining the image.

The calculated transform is then utilised to determine estimated threedimensional co-ordinate centres of each of the circles identified bymarkers. These calculated co-ordinates then identify an estimatedlocation and orientation for the plane corresponding to the surface ofthe calibration sheet relative to the position from which an image hasbeen obtained.

Each pixel in the image obtained by the cameras 10 is then processed inturn to determine where within the plane containing the estimatedpositions of the circle centres, each pixel corresponds. The estimatedcircle centres are then processed in turn and the pixels in an imagecorresponding to points lying within a pre-determined distance from eachcircle centre in the calculated plane are then identified. The greyscale values for each of the pixels in each set are then utilised todetermine an improved estimate of the co-ordinates for the circlecentres. x and y co-ordinates for the positions of the points in theestimated plane including the surface of the calibration sheet eachpixel represents within the set is determined. These calculated x and yco-ordinates are then utilised to estimate an improved estimate of thex,y co-ordinates of the circle centre using the following equations:

$x = \frac{\sum{gx}}{\sum g}$ $y = \frac{\sum{gy}}{\sum g}$

where Σg is the sum of all pixel values in the set identified for aparticular circle centre, Σgx is the sum of the grey scale pixel valuesfor a pixel multiplied by the x co-ordinates for those pixels and Σgy isthe sum of the grey scale pixel values for a pixel multiplied by the yco-ordinates for those pixels, and where the colour of the circle isassociated with a high grey scale value and the colour of the backgroundcorresponding to the calibration sheet is associate with a low greyscale value.

Co-ordinates for the point within the image corresponding to the newestimated circle centres are then determined from these x,y co-ordinatesand these updated estimates of the centres of the marked circles arethen utilised to determine a more accurate estimated transformation toaccount for the location and orientation of the calibration sheet. Theabove process can then be repeated until an accurate estimate of theactual circle centre positions is made and the true transform requiredto account for the relative orientation of the calibration sheet isdetermined.

Using the final determined transform, the expected positions of all ofthe circles on the sheet appearing in the image are then calculated, theportions of the images in the vicinity of each of the estimated circlecentres are then processed individually in the same way as describedabove. For each of the circles a set of pixels is identifiedcorresponding to points within a pre-set distance to the circle centreand then an improved circle centre co-ordinate is calculated using thegrey scale values and co-ordinate values as described above.

When the co-ordinates for all the centres of each of the representationsof the circles on the calibration sheet have been calculated for animage, the relative orientation of the different cameras within thestereoscopic camera system 10 can then be calculated from the relativepositions of these points in the images and the known relative locationsof these circles on the surface of the calibration sheet as is describedin detail in “A Versatile Camera Calibration Technique for High-Accuracy3D Machine Vision Metrology Using Off the Shelf TV Cameras and Lenses”,Roger Tsai, IEEE Journal of Robotics and Automation, Vol. Ra-3, No. 4,August 1987. Further from the relative positions of the points in theindividual images internal camera parameters such as the focal lengthand radial distortion within the camera images can also be determined.

Thus by performing this initial calibration step, the computer 12 isprovided with data which enables the computer to convert stereoscopicimages obtained by the stereoscopic camera system 10 into 3D computerwire mesh models of the surfaces of imaged objects. Further, since theinitial calibration is performed relative to a calibration sheet ofknown dimensions, the computer is also provided with data identifyingthe relationship between distances in the computer models and real worldmeasurements. However, at this stage there is no means by which thecomputer 12 can relate such measurements to the position of theiso-centre of the treatment room or the axes of motion of the mechanicalcouch.

Having determined the relative locations and relevant internalparameters of the cameras of the stereoscopic camera system 10, acalibration phantom 18 is then placed (s2) at the estimated iso-centrefor the treatment room. The phantom 18 in this embodiment comprises amachined cube of known dimensions of a low density material withradiographic attenuation comparable to water and a density comparable tothat of human tissue. In this embodiment, this is achieved by thephantom 18 being a machined polystyrene cube with the surface finish ofthe cube painted to provide optimal imagery for imaging by thestereoscopic camera system 10. In other embodiments other materialscould be used. However in any embodiment the materials used should beselected to facilitate imaging using imaging techniques such as kV,stereo x-ray, MV or portal imaging, Cone Beam CT, and conventional axialand helical CT.

A known asymmetrical arrangement of targets is provided in a fixedorientation within the body of the phantom 18. These targets maycomprise small metallic or ceramic balls. The balls may be made ofsteel, titanium or tungsten or may be made of a ceramic material or anyother material which provides a good contrast to the material formingthe body of the phantom 18 when the phantom is imaged. Most suitablematerials will be materials which have good radiation stability whichare able to withstand considerable radiation dose before undergoingsubstantial change where the relative density and radiographicattenuation of the targets provided within the body of the phantom 18 isselected to assist with the subsequent imaging of the phantom 18. It ispreferable that any phantom 18 contains an asymmetrical arrangement oftargets as the asymmetry of the targets enables the orientation of thephantom 18 to be determined from images of the phantom 18, enabling theorientation of the phantom 18 with respect to the treatment co-ordinatesystem to be determined or confirmed if already known.

Suitable phantoms 18 with an asymmetric arrangement of imaging targetswould include the Penta-Guide phantom (Modus Medical Devices, LondonOntario, Canada) which consists of an acrylic cube of 16 cm size withfive internal air pockets orientated in a unique pattern.

The phantom 18 is located at an estimated location for the iso-centre bybeing placed on an adjustable table 20 mounted on the mechanical couch16. The adjustable table 20 in addition to fixing the location of thephantom 18 on the mechanical couch 16 is arranged to have adjustablefeet so that minor adjustments to the orientation of the phantom can bemade. This is desirable because the axes of motion of a typicalmechanical couch 16 are limited to rotation and lateral, vertical andhorizontal motion and hence motion of a mechanical couch 16 can onlyapproximate changes in for example pitch or roll. In some embodimentsthe adjustable table 20 may include a spirit level which can indicate toa user when the table 20 and hence the phantom 18 is level.

So as not to impact the quality of the radiographic images, from anypotential x-ray/photon beam scatter, the adjustable table 20 isconstructed from materials with low density/radiographic attenuation. Inaddition in this embodiment the table 20 has a clear aperture in thepath of the vertical treatment beam, so as not to adversely affect thequality of the radiographic images obtained when the phantom 18 isirradiated.

In some embodiments, the adjustable table 20 may be arranged so that itcan only be attached to the mechanical couch 16 at a number of fixedlocations and in fixed orientations relative to the surface of themechanical couch 16. Similarly, the adjustable table 20 may also bearranged so that a phantom 18 may only be placed on the table 20 in oneor more fixed orientations. Thus for example the adjustable table 20 mayinclude a recess for receiving the phantom 18 in a particularorientation. The attachment of the adjustable table 20 to the mechanicalcouch 16 and the fixing of the position and orientation of the phantom18 relative to the adjustable table 20 assists with fixing thepositioning and orientation of the phantom relative to the iso-centreand co-ordinate system for the treatment room. In any event theadjustment table 20 should be constructed so as not to slip or move onthe surface of the mechanical couch 16.

In some embodiments, the adjustable table may incorporate a micrometerpositioning tool for precisely moving the phantom laterally,longitudinally and vertically. The advantage of such a positioning toolwould be that it would be easier to make fine amendments to the positionof the phantom using the tool rather than the mechanical couch 16 sincethe relocation would only involve movement of the phantom and not theentire couch 16.

In some embodiments a phantom which incorporated such a tool might beutilized in place of a separate phantom 18 and adjustable table 20. Anexample of such a phantom would be the Varian Exact T Look-Bar (MedTec,Orange City, Iowa, USA) which consists of five markers embedded in atissue equivalent material which is mountable on a couch via a cam lockmechanism which incorporates a micrometer tool.

Having positioned the phantom 18 with its centre point at the estimatedtreatment room iso-centre and if necessary leveled the phantom 18 byadjusting the feet of the adjustable table 20, images of the phantom 18are obtained (s3) using the stereoscopic cameras 10. These images arethen processed by the computer 12 using the previously obtainedinformation about the relative locations and internal parameters of thecameras of the stereoscopic camera system 10 to create a 3D computermodel of the surface of the phantom 18 in the current 3D model space.

The phantom 18 is then (s4) irradiated using the treatment apparatus 14from a variety of angles and images of the projected radiation areobtained in a conventional manner in a similar way to the conventionalWinston-Lutz test using a digital camera 22 arranged at a fixed pointopposite the gantry head of the treatment apparatus 14 irradiating thephantom 18. The images obtained by the digital camera 22 when the gantryhead of the treatment apparatus 14 is orientated in different locationsare then processed by the computer 12 in a conventional way to determinefrom differences between the appearance of the phantom 18 in the imagesand in particular shadows of the targets contained within the phantom 18and compared with the expected locations of shadows of the targetswithin the phantom on the assumption that the phantom 18 is correctlyplaced at the iso-centre. From this processing, the computer 12determines the extent to which the centre of the phantom is offset fromthe true iso-centre of the treatment apparatus 14.

The computer 12 then (s5) proceeds to apply the determined offset to themodelled surface of the phantom 18 to generate a reference surface forthe correct positioning of the phantom 18. Having done so, the computerinstructs the mechanical couch 16 to reposition itself laterally,vertically, longitudinally and rotationally so as to relocate thephantom 18 to the position of the true iso-centre. If the phantom 18 iscorrectly leveled prior to imaging an exact realignment of the phantom18 should be achievable purely through lateral, vertical, longitudinaland rotational transformations.

As the phantom 18 is repositioned, the location of the phantom 18 ismonitored (s6) using the stereoscopic camera system 10 with the computer12 processing images of the phantom 18 to generate a wire mesh models ofthe surface of the phantom 18 as it is repositioned. Any differencesbetween the modelled surface and the original surface to which therequired offset for relocating the phantom 18 to the true iso-centre canbe displayed on the screen of the computer 12 to provide visual feedbackof errors which remain.

Monitoring of the motion of the phantom 18 provides an opportunity forconfirming that the axes of the co-ordinate system being used togenerate wire mesh models corresponds to the axes of the co-ordinatesystem for the mechanical couch 16. More specifically, the analysis ofthe images obtained by the digital camera 22 will provide an offsetwhich is the offset for repositioning the phantom 18 at the trueiso-centre for the treatment room. Instructing the mechanical couch 16to reposition the phantom 18 in a manner corresponding to the offsetshould result in a motion with corresponds which causes the phantom 18to be correctly positioned. In addition the motion of the phantom 18 dueto the repositioning of the couch 16 should also correspond to anequivalent relocation of the surface of the phantom 18 as monitored bythe stereoscopic camera system 10. Additionally, this monitoring of thesurface of the phantom 18 can highlight the existence of any furtherminor adjustments to the position or orientation of the phantom 18 suchas might be achieved by adjusting the adjustable table 20 after thephantom 18 has been repositioned by the mechanical couch 16.

When the phantom 18 located with its centre point at the estimatedposition of the iso-centre, the stereoscopic camera 10 system can thenuse the images of the phantom 18 of known dimensions at a known locationtogether with the known relative locations of the cameras of thestereoscopic camera system 10 to calibrate the co-ordinate system forthe 3D surface models generated by the computer 12 relative to theactual location of the iso-centre. This is typically achieved byredefining the co-ordinate system for the computer model so that theorigin for the computer model co-ordinate system coincides with thecentre of the phantom 18 as determined from the modelled surface of thephantom 18 in its new position.

In embodiments of the present invention, steps S3-S6 can be repeated ifdesired to obtain further confirmation to the relative position of thephantom 18 to the iso-centre. That is to say, after having relocated themechanical couch 16 based on the determined offset based on imagesobtained by the digital camera 22 and monitoring that the phantom 18 hasbeen correctly relocated using the stereoscopic camera system 10, therepositioned phantom 18 could then be re-irradiated and further imagesof the irradiated phantom 18 could then be obtained to confirm that thephantom 18 was indeed located at the iso-centre and if this was not thecase further adjustments in the position of the phantom 18 could bemade.

For additional accuracy in a second iteration, a phantom 18 could beutilised comprising a single target where the phantom 18 comprises acube of known dimensions with a single spherical target embedded in thecube where the sphere is located at the centre of the cube. Such aphantom 18 could then be imaged and adjustments made on the basis ofanalysis of images obtained by the digital camera 22 when the phantom isirradiated. The benefit of such a phantom 18 is that the target spherecould made to be very small and thereby assist with locating theiso-centre with greater accuracy. The disadvantage of such a phantom 18would be that if only a single target sphere is present, images of theobtained phantom 18 will lack information about the orientation of thephantom 18 which can be derived where the phantom 18 contains anasymmetric arrangement of targets.

A suitable test phantom which consists of one central 2 mm radio-opaquemarker embedded in tissue equivalent material would be the Varian cube(Varian Medical Systems, Palo Alto, Calif., USA).

In embodiments where multiple phantoms 18 are utilised, it is beneficialthat an adjustable table 20 is used which permits attachment of aphantom 18 in a single orientation. In such a system, initially aphantom 18 containing multiple targets could be used to identify thelocation of the iso-centre of the treatment apparatus. Using themulti-target phantom would have the advantage that because of theasymmetric arrangement of target spheres is apparent, it is possible todetermine the orientation of the phantom 18 as it appears in imagesobtained by the digital camera 22. Having done so and havingrepositioned the phantom 18 based on analysis of the images, themulti-target phantom 18 could then be replaced with a single targetphantom 18 and final minor adjustments could be made to the calibrationusing the single target phantom. When changing the phantom 18 thestereoscopic camera system 10 could be utilised to ensure that no changein position occurred when the phantoms 18 were exchanged. In suchembodiments, it would be beneficial if the multiple phantoms 18 could bemarked in a manner (e.g. colour coded) which enabled the individualphantoms 18 to be easily distinguished from one another.

In the above embodiments the calibration of a camera system has beendescribed based upon the imaging of a phantom 18 which has been locatedto the iso-centre for a treatment room. It will be appreciated that insome embodiments, calibration could be achieved without physicallyrelocating a phantom 18. More specifically, as described, a phantom 18could be positioned at the estimated location for an iso-centre andimaged using the stereoscopic camera system 18 and then irradiated withimages being obtained using the digital camera 22. The images obtainedby the digital camera 22 could then be processed to determine the offsetof the phantom relative to the actual iso-centre for the treatment room.Similarly, images of the phantom 18 obtained by the stereoscopic camerasystem 10 could be processed to generate a 3D computer model of thephantom at its current location. An adjustment for the co-ordinatesystem for the stereoscopic camera 10 could then be calculated by addingthe transform calculated by processing the 3D model to relocate thecentre of the model of phantom 18 with the origin of the axes for thecomputer model and the offset as determined by processing the imagesobtained by the digital camera 22.

What is claimed is:
 1. A method of calibrating a 3D camera system for use with a radio therapy treatment apparatus, the method comprising: storing a 3D computer model of a surface of a calibration phantom with the center of the modelled phantom located at the origin for the co-ordinate system for the 3D computer model; positioning a calibration phantom with the calibration phantom's center at an estimated location for the iso-center of a radio therapy treatment apparatus; irradiating the calibration phantom using the radio therapy treatment apparatus and determining the relative location of the center of the calibration phantom and the iso-center of the radio therapy treatment apparatus by analyzing images of the irradiation of the calibration phantom utilizing a 3D camera system to obtain images of the calibration phantom and processing images obtained by the 3D camera to generate a further 3D computer model of the surface of the calibration phantom; comparing the modelled surface of the calibration phantom generated on the basis of images obtained by the 3D camera system with the stored model to determine a transformation to make the generated 3D computer model of the surface of the calibration phantom coincide with the stored computer model; and setting the co-ordinate system of the 3D camera system on the basis of the sum of the transformation required to make the generated 3D computer model of the surface of the calibration phantom coincide with the stored computer model and the transformation required to make location of the center of the calibration phantom coincident with the iso-center of the radio therapy treatment apparatus.
 2. The method of claim 1 wherein the calibration phantom comprises a calibration cube and the stored 3D computer model of the surface of a calibration phantom comprises a 3D computer model of a surface of a calibration cube.
 3. The method of claim 2, wherein the calibration phantom comprises an acrylic or polystyrene cube.
 4. The method of claim 2, wherein the calibration phantom contains an arrangement of one or more targets made of a material having good radiation stability.
 5. The method of claim 4 wherein the targets comprise one or more metallic or ceramic balls.
 6. The method of claim 5 wherein the targets arranged asymmetrically within the calibration phantom. 